Tough and weak crystal mixing for low power grinding

ABSTRACT

A resin bond grinding element is composed of a resin bond matrix containing superabrasive particles. The superabrasive particles are an at least 1:1 volume mixture of tough and weak particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The corresponding method involves grinding a workpiece with a resin bond grinding element composed of a resin bond matrix containing an at least 1:1 volume mixture of tough and weak superabrasive particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The mixture of tough and weak superabrasive particles also can be used in metal bond and vitreous bond grinding elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to the manufacture ofabrasive cutting and grinding elements and more particularly to the useof mixed abrasive crystals of different shapes and toughness to achieveoverall lower power consumption in grinding operations.

[0004] Grinding wheels of various shapes, sizes, and composition arewell known in the art. Wheels containing superabrasive materials (e.g.,diamond or cubic boron nitride, CBN) in the edge or outer periphery of acircular grinding wheel or grinding cup also are well-known in the artin sawing, drilling, dressing, grinding, lapping, polishing, and otherabrading applications. For these applications, the grit typically issurrounded in a matrix of a metal, such as Ni, Cu, Fe, Co, Sn, W, Ti, oran alloy thereof, or of a resin, such as phenol formaldehyde or otherthermosetting polymeric material. By attaching the matrices to a body orother support, tools may be fabricated having the capability to cutthrough such hard, abrasive materials as steels, superalloys, ceramics,and cermets.

[0005] To minimize the heat generated in the grinding operation, whichcan thermally damage the workpiece, it is generally desired to minimizethe power required to remove material. This power is consumed by acombination of friction caused by the actual cutting of material by thesuperabrasive grains, and friction caused by rubbing of the wheel bondmaterial with the workpiece. By minimizing the total friction in thematerial removal operation, the working temperature can be minimized,thereby minimizing the chance of thermal damage to the workpiece. Thepresent invention describes a way of minimizing the power consumption.

BRIEF SUMMARY OF THE INVENTION

[0006] A resin bond grinding element is composed of a resin bond matrixcontaining superabrasive particles. The superabrasive particles are anat least 1:1 volume mixture of tough and weak particles, wherein thereis at least about 10% difference in toughness between the toughparticles and the weak particles. The mixture of tough and weaksuperabrasive particles also can be used in metal bond and vitreous bondgrinding elements.

[0007] The corresponding method involves grinding a workpiece with aresin bond grinding element composed of a resin bond matrix containingan at least 1:1 volume mixture of tough and weak superabrasiveparticles, wherein there is at least about 10% difference in toughnessbetween the tough particles and the weak particles. The tough/weaksuperabrasive particle mixture lowers the power required in grinding,thereby minimizing the temperature of grinding and ultimately minimizingthe chance of thermal damage to the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a fuller understanding of the nature and advantages of thepresent invention, reference should be had to the following detaileddescription taken in connection with the accompanying drawings, inwhich:

[0009]FIG. 1 displays the power consumption, as measured by specificenergy as a function of abrasive concentration in the wheel for datareported in Example 1;

[0010]FIG. 2 displays the grinding power or specific energy as afunction of toughness of the wheel as reported in Example 2; and

[0011]FIG. 3 is a schematic illustration of the protrusion of the weakand tough crystals above the bond wheel.

[0012] The drawings will be described in detail below.

DETAILED DESCRIPTION OF THE INVENTION

[0013] It is well known that a lower concentration of crystals in abonded wheel leads to lower power consumption in operation. This can beexplained by understanding that the majority of power is consumed in thefriction generated in the cutting operation (crystal creating a chip inthe workpiece) and in the frictional rubbing of the wheel against theworkpiece which does not lead to material removal. The sum of thesefrictional processes causes the grinding operation to be a hightemperature process. FIG. 1 shows the power consumption (as measured bythe specific energy) as a function of abrasive concentration in thewheel, as reported in Example 1. As the abrasive concentration goesdown, it is known that the grinding power will decrease (lower rate of“scratching” of the workpiece), but each diamond crystal will have toremove a bigger “chunk” of material for a given overall material removalrate, so the surface finish becomes poorer.

[0014] It also is known that grinding power or specific energy decreaseswith crystal toughness. This is demonstrated in FIG. 2, which shows thata stronger or tougher crystal also leads to a lower grinding power, asreported in Example 2. Combine the tougher crystals with weaker crystals(acting almost as a “filler” to the bond system), and the operatorshould see the effect. That is, if the operator mixes a weaker crystalinto the mix, the weak crystals will break down and not have a bigimpact on power, but they should impact the finish by “smoothing” outthe poor finish created by the lower concentration of tougher crystals.As a result, a satisfactory finish will be realized with lower overallpower.

[0015] The present invention, then, uses the foregoing two effects tocreate an abrasive blend, which will grind with lower power. Lower powergrinding is accomplished by blending a large amount of high toughnesscrystals with a smaller amount of low toughness crystals to give anoverall abrasive blend. The high toughness crystals will do most of thecutting while the low toughness crystals will tend to fracture and notwork very hard. The effective concentration of the mixture, then, willbe lower than the equivalent concentration of either high or lowtoughness crystals and, more importantly, the power will be lower. Inaddition, since the high toughness crystals will be dominating the work,they will lead to lower grinding power. Finally, the low toughnesscrystals will act to “smooth” out the surface finish leading to a goodfinish at low power consumption.

[0016] In general, the maximum ratio of strong and weak crystals shouldbe on the order of about 1:1 in crystal volume; so if the weak crystalsare smaller, there would be more of them. However, a range of 10:1 to1:1 (by volume) of strong to weak crystals should produce the desiredaffect.

[0017] As for toughness, the “TI” test (toughness index) is an arbitrarytest. The toughness index (TI) is measured at room temperature. In manycases, the tougher the crystal, the longer the life of the crystal in agrinding or machining tool and, therefore, the longer the life of thetool. This leads to less tool wear and, ultimately, lower overall toolcost. For the present invention, the difference between “Tough” and“Weak” crystals should be greater than about a 10% difference on the TIscale, with something more like about 30%-90% difference being morecommon.

[0018] With respect to high and low toughness crystals, it is generallyknown that inclusions in the particles lower toughness or strength,while nitrogen in the lattice improves the strength. See for example,Jackson, et al., “Influence of substitutional nitrogen in syntheticsaw-grade diamond on crystal strength’, J. Mater. Res., Vol. 12, No. 6,p. 1646 (1997).

[0019] The diamond particles can be natural or synthetic. Syntheticdiamond most often is used in grinding operations. Synthetic diamond canbe made by high pressure/high temperature (HP/HT) processes, which arewell known in the art. The particle size of the diamond is conventionalin size for resin-bond or other grinding wheels. Generally, the diamondgrit can range in particle size from about 400 mesh (37 microns) upwardsto 40 mesh (425 microns). Narrow particle size distributions can bepreferred according to conventional grinding technology.

[0020] Cubic boron nitride (CBN) also can be used in accordance with thepresent invention, as CBN crystals too can be classified as tough orweak according to the TI scale. A blend of tough and weak CBN crystalslikewise will produce the desired affect. CBN also can be made by HP/HTtechniques, as is well known by those skilled in the art.

[0021] The coating of diamond and cubic boron nitride (CBN) with nickel,nickel-phosphorous alloys, cobalt, cobalt-phosphorous alloys, copper,and various combinations thereof is a standard procedure in the industryfor enhancing retention of the abrasives in resin and other bondedtools, and for enhancing the grinding operation.

[0022] The patent literature is replete in the coating field. See, forexample, U.S. Pat. Nos. 2,411,867; 3,779,727; 3,957,461; 3,528,788;3,955,324; 4,403,001; and 4,521,222; British Pat. No. 1,344,237; andGerman Pat. No. 2,218,932. U.S. Pat. Nos. 4,024,675 and 4,246,006 formaggregates of diamond grit in a metal matrix that includes silver andU.S. Pat. No. 4,239,502 dips diamond or cubic boron nitride (CBN) in amolten silver/manganese/zirconium brazing alloy. Some attempts have beenmade to enhance the adhesion of the abrasive-coating interface bydeposition of a carbide-forming element under the Ni, Co, or Cu coating.(U.S. Pat. Nos. 5,232,469 and 5,024,680). Some attempts have also beenmade at improving the coating-resin interface, but all of these involveincreasing the mechanical forces by roughening the surface of thecoating (see for example U.S. Pat. Nos. 3,650,714 and 4,435,189; andIrish Patent No. 21,637). The coatings enhance the retention of thecrystals in the bond by providing greater surface texture (also helpwith heat dissipation, lubrication, other minor factors). The advantagesof the invention are realized whether or not the crystals are coated.

[0023] The resin most frequently used in resin bond grinding wheels is aphenol-formaldehyde reaction product. However, other resins or organicpolymers may be used, such as, for example, melamine or ureaformaldehyde resins, epoxy resins, polyesters, polyamides, andpolyimides. Such grinding wheels are made from the abrasive mixture bytheir mixing with resin powders and other additives (SiC, Cu powders),pressing the mixture in a mold, and heating to cure the resin.

[0024] The tough and weak abrasive particle mix also can be combinedwith vitreous matrix composite materials. The mixture then can besintered or hot-pressed following procedures common in the vitreous bondart. For vitreous bond grinding wheels, for example, the abrasiveparticle mix is mixed with SiO₂, B₂O₃, Na₂O, CaO, MgO, or other similarglass forming material(s), and hot pressed.

[0025] The tough and weak abrasive particle mix further can beincorporated into a metal bond grinding wheel. For making these wheels,the tough and weak abrasive particle mix is added to a matrix of ametal, such as Ni, Cu, Fe, Co, Sn, W, Ti, or an alloy thereof, andprocessed conventionally.

[0026] Concentration of coated diamond and fabrication of all of theforegoing wheels is conventional and well known in that art. Broadly,such concentrations range from about 25 to 200 (100 concentrationconventionally being defined in the art as 4.4 carats/cm³ with 1 caratequal to 0.2 g, wherein the concentration of diamond grains is linearlyrelated to its carat per unit volume concentration). Preferably, theconcentration of diamond grit ranges from about 50-100.

[0027] Grinding wheels can be disc shape or cup shape and can contain asecondary distribution of silicon carbide or other secondary abrasiveparticles without detrimentally affecting the performance of thegrinding element containing the tough/weak mixed abrasive particles. Ina typical preparation of a resin bond grinding wheel, for example, amixture of granulated resin, abrasive particle mixture, and optionalfiller is placed in a mold. A pressure appropriate to the particularresin, usually several thousand pounds per square inch (several tens ofthousands of Kilo Pascals, KPa), is applied, and the mold is heated to atemperature sufficient to make the resin plastically deform (and curewhen the resin is heat-curable). Techniques for fabricating metal andvitreous bond wheels also are well known in the art.

[0028] While the invention has been described with reference to apreferred embodiment, those skilled in the art will understand thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. In this application all units are in the metric system and allamounts and percentages are by weight, unless otherwise expresslyindicated. Also, all citations referred herein are expresslyincorporated herein by reference.

EXAMPLE 1

[0029] This example demonstrates the increase in specific energy withincreasing wheel abrasive concentration. The measurements were made inthe surface grinding of M-2 tool steel (M2 HSS, HRC 60-62) using a 1A1grinding wheel. The wheel was dressed by plunging into a soft aluminumoxide dressing stick. The grinding conditions included: 6000 sfpm wheelspeed, 50 fpm table speed, downfeed of 0.0015″ per pass, and a coolantflow of 5 gpm at 20 psi. The measured specific energy is in units ofW-hr/cm³. The abrasive was a 60% Ni coated CBN (GE brand “Type-II”, GESuperabrasives, Worthington, Ohio) of mesh size 120/140 and the wheelwas a phenolic resin type bond matrix. The only variable changing in thetests was the abrasive concentration, which is defined as the volumefraction of abrasive to wheel. A 100 concentration wheel is 24 volume-%,while a 50 concentration wheel is 12 volume-%. The data recorded is setforth below and illustrated in FIG. 1: TABLE 1 CONCENTRATION SpecificEnergy 100 14.3 80 12.8 65 11.6 50 10.3

[0030] The data recorded shows that higher wheel concentrations lead toa higher specific energy in grinding. This may be interpreted asfollows. A higher concentration of abrasive translates into a largernumber of crystals per volume of wheel and, therefore, a larger numberof cutting points. Each cutting point is a contributor to the grindingpower. As the number of cutting points increases, the required power, orenergy of grinding, increases. But the finish on the part is better witha higher concentration on the wheel, because the wheel has more cuttingpoints, each of which takes a smaller “scoop” of material out of theworkpiece. This is true generally, because most grinding operations arerun at a fixed material removal rate. So, the tradeoff is a good surfacefinish for a higher grinding power (and, therefore, higher temperatureexposure of the workpiece). This interpretation is independent ofwhether you have a resin, metal, or vitreous bonded wheel.

EXAMPLE 2

[0031] This example demonstrates the decrease in specific energy withincreasing crystal toughness. The tests were done with a 1A1 vitrifiedgrinding wheel in the creepfeed grinding of M2 HSS. In this case, thediamond was uncoated (a coating is not needed for a vitreous bond wheel)and of mesh size 120/140. The wheel concentration was 150 (36 volume-%)and was dressed by the same method used in Example 1. The grindingconditions were: 6000 sfpm wheel speed, 10 ipm table speed (creepfeedmode), and depth of cut of 0.040″. The grinding was done with a waterbased coolant (24 gpm at 95 psi). The data recorded is set forth belowand displayed in FIG. 2: TABLE 2 Power (W/cm) Toughness Run Repeated Run49.3 4752 4748 40.8 5457 5190 64.9 3018 2961 84.8 2841 2833

[0032] These results can be explained as follows. A stronger crystal canmaintain a higher protrusion above the bond in a grinding wheel. By thecrystal sitting at a higher protrusion, there is more room for thecutting chips to be washed away from the workpiece, and there is moreroom for the coolant to get to the cutting area. Overall, these resultin the overall friction generated by the chips and workpiece rubbingagainst the bond being decreased. An analogy is that a wheel with toughcrystals which protrude high above the bond will be “sharper”, like asharpened saw, than a wheel with weak crystals which are not protrudingvery high above the bond. This is why the overall grinding powerdecreases for higher strength (toughness) crystals.

[0033] The invention takes the value of both of these observations togive an overall lower grinding power while still maintaining a goodsurface finish. If the wheel has a mixture of tough and weak crystals,the weak crystals will protrude to a lower level than the toughcrystals, thereby leading to a lower number of the highly effectivecutting points at the grinding surface. However, there will be somecutting action of these weaker crystals, which will act to smooth outthe surface finish of the workpiece. A schematic of the protrusion abovebond 10 is illustrated in FIG. 3 for weak crystals 12 and tough crystals14.

[0034] Thus, by replacing some of the tough crystals in a wheel withweak crystals, the invention demonstrates that the cutting friction willbe lowered, but the overall finish of the part will be nominally thesame. This is a method for lowering the power of grinding throughcrystal mixing.

1. In a bond grinding element composed of a bond matrix containingsuperabrasive particles, the improvement which comprises: saidsuperabrasive particles comprising an at least 1:1 volume mixture oftough and weak particles, wherein there is at least about 10% differencein toughness between said tough particles and said weak particles. 2.The grinding element of claim 1, wherein said bond matrix is one or moreof a resin bond matrix, a vitreous bond matrix, or a metal bond matrix.3. The grinding element of claim 1, wherein said abrasive particles areone or more of diamond or cubic boron nitride (CBN).
 4. The grindingelement of claim 1, wherein said volume ratio of tough to weak particlesranges from about 10:1 to 1:1.
 5. The grinding element of claim 1,wherein there is a difference in toughness between said tough particlesand said weak particles ranging from between about 30% and 90%.
 6. Thegrinding element of claim 2, wherein said bond matrix is a resin bondmatrix of one or more of a phenol-formaldehyde resins, melamineformaldehyde reins, urea formaldehyde resins, epoxy resins, polyesters,polyamides, and polyimides.
 7. The grinding element of claim 6, whereinsaid abrasive particle is diamond.
 8. In a method for grinding aworkpiece with a bond grinding element composed of a bond matrixcontaining superabrasive particles, the improvement which comprises:providing said grinding element to contain superabrasive particlescomprising an at least 1:1 volume mixture of tough and weak particles,wherein there is at least about 10% difference in toughness between saidtough particles and said weak particles.
 9. The method of claim 8,wherein said bond matrix is one or more of a resin bond matrix, avitreous bond matrix, or a metal bond matrix.
 10. The method of claim 8,wherein said abrasive particles are one or more of diamond or cubicboron nitride (CBN).
 11. The method of claim 8, wherein said volumeratio of tough to weak particles ranges from about 10:1 to 1:1.
 12. Themethod of claim 8, wherein there is a difference in toughness betweensaid tough particles and said weak particles ranging from between about30% and 90%.
 13. The method of claim 9, wherein said bond matrix is aresin bond matrix of one or more of a phenol-formaldehyde resins,melamine formaldehyde reins, urea formaldehyde resins, epoxy resins,polyesters, polyamides, and polyimides.
 14. The method of claim 13,wherein said abrasive particle is diamond.